Definition:
An irregular variation of ocean current that from January to Marchflows off the west coast of South America, carrying warm,low-salinity, nutrient-poor water to the south. It does not usuallyextend farther than a few degrees south of the equator, butoccasionally it does penetrate beyond 12 degrees S, displacing therelatively cold Peru Current. The effects of this phenomenon aregenerally short-lived, and fishing is only slightlydisrupted. Occasionally (in 1891, 1925, 1941, 1957 - 58, 1965, 1972 -73, 1976, and 1982 - 83), the effects are major and prolonged. Underthese conditions, sea surface temperatures rise along the coast ofPeru and in the equatorial eastern Pacific Ocean and may remain highfor more than a year, having disastrous effects on marine life andfishing. Excessive rainfall and flooding occur in the normally drycoastal area of western tropical South America during theseevents. Some oceanographers and meteorologists consider only themajor, prolonged events as El Nino phenomena rather than the annuallyoccurring weaker and short-lived ones. The name was originally appliedto the latter events because of their occurrence at Christmas time.("http://cdiac.esd.ornl.gov/glossary.html")

Interacting parts of a single global system of climatefluctuations. ENSO is the most prominent known source of interannualvariability in weather and climate around the world, though not allareas are affected. The Southern Oscillation (SO) is a global-scaleseesaw in atmospheric pressure between Indonesia/North Australia, andthe southeast Pacific. In major warm events El Nino warming extendsover much of the tropical Pacific and becomes clearly linked to the SOpattern. Many of the countries most affected by ENSO events aredeveloping countries with economies that are largely dependent upontheir agricultural and fishery sectors as a major source of foodsupply, employment, and foreign exchange. New capabilities to predictthe onset of ENSO event can have a global impact. While ENSO is anatural part of the Earth's climate, whether its intensity orfrequency may change as a result of global warming is an importantconcern.From: "http://earthobservatory.nasa.gov/Library/glossary.php3"